The present invention relates to electromagnetic communication between the ground and an orbiting satellite and, more particularly, to a feed assembly, for a ground station antenna, that supports communication with satellites that transmit and receive in several frequency bands and/or using linear and circular polarizations.
FIGS. 1A and 1B shows a typical parabolic dish antenna 10 for communicating with a communication satellite such as a Fixed Service Satellite (FSS). Antenna 10 includes a parabolic dish 12 and a Low Noise Block downconverter Feed horn (LNBF) 14 supported by supports 16 at the focus of dish 12. Dish 12 is mounted on a mount 18. FIG. 1A is a perspective view of antenna 10. FIG. 1B is a frontal view of dish 12 and LNBF 14. LNBF 14 includes a Low Noise Block (LNB) with two orthogonal receive dipoles 20 shown in FIG. 1B in phantom. Each dipole receives Ku-band signals from the FSS at which antenna 10 is aimed.
An FSS is a geostationary satellite whose transponders transmit and receive linearly polarized radio waves in the Ku-band. One transponder of a transponder pair transmits and receives horizontally polarized waves. The other transponder of the transponder pair transmits and receives vertically polarized waves. LNB dipoles 20 are intended for receiving signals in respective allocated frequency segments from respective transceivers of the FSS: the horizontal dipole antenna 20 is for receiving signals from the transponder that transmits horizontally polarized waves and the vertical dipole antenna 20 is for receiving signals from the transponder that transmits vertically polarized waves. If the FSS is at the same longitude as a stationary antenna 10, then when dish 12 is aimed at the FSS by appropriate adjustment of mount 18 in azimuth and elevation, the horizontal LNB dipole 20 is aligned with the horizontal polarization direction of the FSS and the vertical LNB dipole 20 is aligned with the vertical polarization of the FSS. If the FSS is not at the same longitude as a stationary antenna 10 then the polarization directions of the FSS are tilted with respect to LNB dipoles 20 and dish 12 must be rotated, as indicated by an arrow 22 in FIG. 1B, to align LNB dipoles 20 with the polarization directions of the FSS.
If antenna 10 is stationary, then dish 12 only needs to be rotated once and then fixed in place on mount 18. If antenna 10 is mounted on a moving platform such as a truck, a boat, an aircraft or some other vehicle, the orientation of dish 12 must be adjusted continuously to keep dish 12 pointed at the FSS and to keep LNB dipoles 20 aligned with the polarization directions of the FSS. Even if antenna 10 is stationary, if antenna 10 communicates with a satellite that is not in a geosynchronous obit, dish 12 must be adjusted continuously to keep dish 12 pointed at the satellite and to keep LNB dipoles 20 aligned with the satellite's polarization directions. Hsiung, in U.S. Pat. No. 6,377,211, teaches an antenna aiming apparatus for keeping an antenna that is mounted on a moving vehicle properly aligned with a satellite in a non-geosynchronous orbit. U.S. Pat. No. 6,377,211 is incorporated by reference for all purposes as if fully set forth herein.
U.S. patent application Ser. No. 12/555,007, which is incorporated by reference for all purposes as if fully set forth herein, teaches a LNBF that makes it unnecessary to rotate dish 12 as a whole, in the directions indicated by arrow 22, to keep LNB dipoles 20 aligned with the polarization directions of the satellite with which antenna 10 communicates.
FIGS. 2A-2D illustrate two embodiments 30 and 31 of a LNBF of U.S. Ser. No. 12/555,007. FIG. 2A is a side view of LNBF 30 showing that LNBF 30 includes, in series, a feed horn 48, a waveguide 50 and a LNB 35. FIG. 2B is a side view of LNBF 31 showing that LNBF 31 includes, in series, feed horn 48, waveguide 50 and an Orthogonal Mode Transducer (OMT) 36. Waveguide 50 includes a rotating distal section 32 and a fixed proximal section 34. FIG. 2C, a cross section of LNBF 30 through section A-A, shows that rotating distal section 32 of LNBF 30 includes a quarter-wavelength dielectric slab polarizer 42. FIG. 2D, a cross section of LNBF 30 through section B-B, shows that fixed proximal section 34 of LNBF 30 includes a quarter-wavelength dielectric slab polarizer 44. Also shown in phantom in FIG. 2D are the orientations of the horizontal dipole 38 and the vertical dipole 40 of LNB 35. Slab 44 is fixed at a 45-degree angle to both horizontal dipole 38 and vertical dipole 40. OMT 36 includes, instead of two orthogonal dipoles, a horizontal port 39 that corresponds to dipole 38 and a vertical port 41 that corresponds to dipole 39.
In general, a single quarter-wavelength dielectric slab polarizer that is placed at a 45-degree angle to a linearly polarized electromagnetic wave, transverse to the direction of propagation of the linearly polarized electromagnetic wave, transforms the linearly polarized electromagnetic wave to a circularly polarized electromagnetic wave. Appropriate rotation of just rotating distal section 32, as indicated by an arrow 46 in FIG. 2C, suffices to keep LNB dipoles 38 and 40 aligned with the polarization directions of the satellite with which an antenna that includes LNBF 30 communicates. Specifically, distal section 32 is rotated to place slab 42 at a 45-degree angle to the polarization directions of the satellite. Distal section 32 transforms the linearly polarized signal from the satellite to a circularly polarized signal, and fixed proximal section 34 transforms the circularly polarized signal to a linearly polarized signal that is aligned correctly with the appropriate LNB dipole 38 or 40. Mathematical details are provided in U.S. Ser. No. 12/555,007.
To minimize reflections in waveguide 50, slabs 42 and 44 should be tapered in the direction of propagation, as shown in FIG. 3. The lengths A and B should satisfy 2A+B≈0.25λ/√∈, where λ is the wavelength of the electromagnetic signal in free space and ∈ is the dielectric constant of the dielectric material of slabs 42 and 44. Length C is tuned for optimal matching of the propagating wave through waveguide 50. Typical values of A, B and C for a Ku-band LNBF 30 are 2 mm, 4 mm and 4 mm, respectively. The dielectric material of slabs 42 and 44 should be of low loss tangent at the operating frequency, e.g. Plexiglas™ (polymethyl methacrylate).
FIG. 4, which is adapted from FIG. 2 of U.S. Pat. No. 6,377,211, is a simplified block diagram of a mechanism for pointing a parabolic dish antenna, that includes LNBF 31 and that is mounted on a moving vehicle, at a geostationary earth satellite while rotating distal section 32 to keep OMT ports 39 and 41 aligned with the polarization directions of the satellite. A Global Positioning System (GPS) receiver 110 mounted on the vehicle receives signals from GPS satellites in a known manner and produces signals that represent vehicle position, the current time (coordinated Universal Time or UPC) and a one-pulse-per-second timing pulse, all of which are applied to a Digital Signal Processor (DSP) 112. The vehicle position information includes latitude, longitude and altitude. A vehicle speed sensor 114 produces signals representing the speed of the vehicle, which are applied to DSP 112. DSP 112 also receives signals representing vehicles roll, inclination (pitch) and azimuth angle (yaw) from (an) appropriate sensor(s) 116 mounted on the vehicle. One such sensor is the Crossbow Model HDX-AHRS, available from Crossbow Technology, Inc. of San Jose Calif., that senses roll, inclination and azimuth angle, and that includes a three-axis magnetometer to make a true measurement of magnetic heading. The azimuth information may be in the form of signals representing vehicle yaw relative to magnetic north; magnetic correction then can be performed in DSP 112 based on the location information from GPS receiver 110 together with stored magnetic declination data. GPS receiver 110, orientation sensor(s) 116 and speed sensor 114 provide DSP 112 with data at an update rate faster than once per second, thereby allowing the antenna pointing system to have a near-real-time response.
The location of the satellite also is stored in DSP 112. DSP 112 processes the sensor signals relative to the location of the satellite to produce antenna drive or control signals, which are applied to the drive motors of the parabolic dish antenna, including a motor for rotating distal section 32, to keep LNBF 31 pointed at the satellite and to rotate distal section 32 to keep OMT ports 39 and 41 aligned with the polarization directions of the satellite.
It also is known to concentrically nest two or more waveguides, of a LNBF, that are tuned to two or more respective frequency bands, so that the ground station antenna can communicate with a satellite that transmits and receives in more than one frequency band without having to swap an LNBF of one band for an LNBF of another band. See, for example, West, U.S. Pat. No. 7,102,581, which is incorporated by reference for all purposes as if fully set forth herein.
It is shown in U.S. Ser. No. 12/555,007 that LNBF 30 can be used for communicating with a satellite that transmits and receives circularly polarized radio waves if slab 42 is kept at a 90 degree angle to slab 44. This is not the case with LNBF 31. It would be highly advantageous to have a LNBF, in which the proximal end of the waveguide is coupled to an OMT, and that can be used for communicating both with satellites that transmit and receive linearly polarized radio waves and with satellites that transmit and receive circularly polarized radio waves.